Replace a range of lines in a file by line number. More token-efficient than edit_file when you already know the line numbers from find_in_file. Only works within allowed directories.
AI agents use replace_lines to create or update resources in FilesystemMCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your FilesystemMCP environment.
This tool modifies file contents by replacing specified line ranges, making it a Write operation (reversible data modification). Severity is high because unintended line replacements could corrupt critical configuration files, source code, or data files, with significant blast radius if an agent replaces wrong lines due to miscalculation. Confidence is high given explicit description of the replacement functionality.
From the tool's definition Tool description states "Replace a range of lines in a file by line number" - a modification operation. Works "within allowed directories" indicating scope constraints but confirming file modification capability.
Attacks that exploit this kind of access
Replace a range of lines in a file by line number. More token-efficient than edit_file when you already know the line numbers from find_in_file. Only works within allowed directories. It is categorised as a Write tool in the FilesystemMCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Filesystem MCP server in PolicyLayer and add a rule for replace_lines: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches FilesystemMCP. Nothing to install.
replace_lines is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the replace_lines rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for replace_lines. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
replace_lines is provided by the Filesystem MCP server (rijadalisic/filesystemmcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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